High purity silica, with or without selected dopants, is used for optical fiber waveguides due to its high transmission quality and inherent strength. Fibers made from these materials can demonstrate high optical clarity and achieve low transmission losses. This permits the use of optical waveguides of great length, frequently without the need for specialized repeaters or amplifiers, as they can provide low loss and low dispersion. Fibers are also useful over short distances for selected applications.
Optical fibers are thus finding use in a number of specialized applications such as long distance, secure, data links for military communications, data buses for satellite and space vehicle systems and real time imaging systems. Such imaging systems includes optical fiber systems for monitoring environmental and experimental conditions around nuclear reactor facilities, and optical fiber data links for plasma fusion reactors where extremely high electromagnetic fields can degrade electrical transmission systems.
Unfortunately, optical waveguides are damaged by exposure to ionizing radiation, such as x-rays, gamma rays or high energy particles. High purity silica core fibers usually display less damage than silicas with dopants in the core. For example, transmission of high purity silica core fibers can be degraded by several decibels per kilometer (dB/km) at 10 kilorad (krad) dose at a wavelength near 800 nanometers (nm). Fibers with extremely low-OH concentrations are required for long wavelength communication applications and are frequently more sensitive to radiation than high-OH materials. Although the increase in transmission loss diminishes with a relaxation period on the order of seconds to hours after irradiation ceases, some permanent damage remains in many types of fiber.
Hydrogen has been identified for over two decades as a significant factor in controlling the performance of silica materials in radiation environments. It has been previously demonstrated that radiation-induced attenuation in bulk silica samples can be reduced by introduction of hydrogen into the silica. Some of the earliest observations of hydrogen effects in fibers included the ease by which molecular hydrogen diffuses through the small dimensions of optical fibers and measurements demonstrated that increases in hydrogen concentration within the fiber core could lead to increased attenuation at certain wavelengths. Efforts have also been directed towards hermetically sealing the surface of optical fibers to significantly reduce diffusion of hydrogen.
In optical fibers, improved performance in ionizing radiation environments with introduction of hydrogen into the fiber was first noted by Nagasawa et al. in Japanese Journal of Applied Physics, vol. 24, pp. 1224-1228 (1985) where it was observed that hydrogen permeation into the fiber optic either prior to or following irradiation suppressed radiation-induced absorption.
Pre-irradiation of pure silica core optical fibers has been observed to frequently lead to improved performance in a subsequent radiation exposure (see, e.g., Lyons et al., SPIE, vol. 1174, pp. 2-19, 1990). Such a result may occur, for example, due to radiation-induced annealing whereby defects are effectively healed thru relaxation of neighboring lattice atoms into configurations resulting in stronger bonds, or in high-OH fibers, the radiation may itself release hydrogen, which could then diffuse to and bond into broken bonds. Other explanations may be possible.
While the previous efforts to provide radiation resistant optical fibers have made much progress, research has continued into this area. Combined treatments, with simultaneous pre-irradiation of hydrogen-impregnated fibers, have not yet been reported.
Accordingly, it is an object of this invention to provide a process of preparing optical fibers having enhanced radiation resistance utilizing a combined treatment with simultaneous pre-irradiation of a hydrogen-impregnated fiber.